Implantable medical electrical stimulation and/or sensing leads are well known in the fields of cardiac stimulation and monitoring, including cardiac pacing and cardioversion/defibrillation, and in other fields of electrical stimulation or monitoring of electrical signals or other physiologic parameters. In the field of cardiac stimulation and monitoring, endocardial leads are placed through a transvenous route to locate one or more sensing and/or stimulation electrode along or at the distal end of the lead in a desired location in a chamber of the heart or a blood vessel of the heart. In order to achieve reliable sensing of the cardiac electrogram and/or to apply stimulation that effectively paces or cardioverts the heart chamber, it is necessary to accurately position the electrode surface against the endocardium or within the myocardium at the desired site and fix it during an acute post-operative phase until fibrous tissue growth occurs.
The pacemaker or defibrillator implantable pulse generator (IPG) or the monitor is typically coupled to the heart through one or more of such endocardial leads. The proximal end of such leads typically is formed with a connector which connects to a terminal of the IPG or monitor. The lead body typically comprises one or more insulated conductive wire surrounded by an insulating outer sleeve. Each conductive wire couples a proximal lead connector element with a distal stimulation and/or sensing electrode. An endocardial cardiac lead having a single stimulation and/or sensing electrode at the lead distal end and a single conductive wire is referred to as a unipolar lead. An endocardial cardiac lead having two or more stimulation and/or sensing electrodes at the lead distal end and two or more conductive wires is referred to as a bipolar lead or a multi-polar lead, respectively.
In order to implant an endocardial lead within a heart chamber, a transvenous approach is utilized wherein the lead is inserted into and passed through the subclavian, jugular, or cephalic vein and through the superior vena cava into the right atrium or ventricle. An active or passive fixation mechanism is incorporated into the distal end of the endocardial lead and deployed to maintain the distal end electrode in contact with the endocardium position. Considerable effort has been undertaken to develop electrode passive and active fixation mechanisms that are simple to use and are reliable. Passive fixation mechanisms do not invade the myocardium but cooperate with cardiac tissue or structures to locate the electrode against the endocardium. The most successful passive fixation mechanism comprises a plurality of soft, pliant tines that bear against cardiac structure surfaces, e.g. the trabeculae in the right ventricle and the atrial appendage, to urge the distal tip electrode against the endocardium. Active fixation mechanisms are designed to penetrate the endocardial surface and lodge in the myocardium without perforating through the epicardium or into an adjoining chamber. The most widely used active fixation mechanism employs a sharpened helix, which typically also constitutes the distal tip electrode. Typically, some sort of shroud or retraction mechanism is provided to shield the helix during the transvenous advancement into the desired heart chamber from which the helix can be advanced and rotated when the desired site is reached to effect a penetrating, screw-in fixation. In one manner or another, the helix is adapted to be rotated by some means from the proximal end of the lead outside the body in order to screw the helix into the myocardium and permanently fix the electrode.
More recently, endocardial pacing and cardioversion/defibrillation leads have been developed that are adapted to be advanced into the coronary sinus and coronary veins branching therefrom in order to locate the distal electrode(s) adjacent to the left ventricle or the left atrium. The distal end of such coronary sinus leads is advanced through the superior vena cava, the right atrium, the valve of the coronary sinus, the coronary sinus, and into a coronary vein communicating with the coronary sinus, such as the great vein. Typically, coronary sinus leads do not employ any fixation mechanism and instead rely on the close confinement within these vessels to maintain each electrode at a desired site.
The heart beats approximately 100,000 times per day or over 30 million times a year, and each beat stresses at least the distal end portion of the lead. Over the years of implantation, the lead conductors and insulation are subjected to cumulative mechanical stresses, as well as material reactions as described below, that can result in degradation of the insulation or fractures of the lead conductors with untoward effects on device performance and patient well being. The endocardial lead body is subjected to continuous stretching and flexing as the heart contracts and relaxes and is formed to be highly flexible and durable. It is necessary to temporarily stiffen the lead conductor sire to advance the lead through these blood vessels and to locate the distal electrode(s) at the desired site.
Early implantable, endocardial and epicardial, bipolar cardiac pacing leads of the type disclosed in U.S. Pat. No. 3,348,548 placed the separate coiled wire conductors in a side by side configuration and incorporated a lumen for receiving a stiffening stylet inside the lumen of at least one of the conductor coils. This design was replaced by a coaxial configuration of the type shown in U.S. Pat. No. 3,788,329 wherein the separate coiled wire conductors are wound in differing diameters separated from one another by tubular insulating sheaths and extend coaxially about a central lumen for receiving the stiffening stylet. The stiffening wire or stylet is advanced through a proximal connector pin opening to stiffen the lead body during the transvenous introduction. Most current endocardial cardiac leads employ multi-filar, parallel-wound, coiled wire conductors electrically connected in common in an electrically redundant fashion as a single polarity lead conductor in each of the unipolar, bipolar and multi-polar lead configurations. Such redundant coiled wire conductors of bipolar and multi-polar lead bodies are coaxially arranged about the stiffening stylet receiving lumen and insulated from one another by coaxially arranged insulating sheaths separating each coiled wire conductor from the adjacent coiled wire conductor(s).
In the implantation of a cardiac device of the types listed above, and in the replacement of previously implanted cardiac leads, two or more transvenous cardiac leads are typically introduced through the venous system into the right chambers or coronary sinus of the heart. It has long been desired to minimize the diameter of the transvenous cardiac lead body to facilitate the introduction of several cardiac leads by the same transvenous approach. Moreover, a number of multi-polar, endocardial cardiac leads have been designed to accommodate more than two electrodes or to make electrical connection with other components, e.g., blood pressure sensors, temperature sensors, pH sensors, or the like, in the distal portion of the lead. The increased number of separate polarity coiled wire conductors is difficult to accommodate in the conventional coaxial coiled wire conductor winding arrangement employing tubular insulating sheaths to separate the coil wire conductors of differing diameters having a desired overall lead body outer diameter.
This need for increased numbers of lead conductors in the lead body has led to the development of separately insulated, coiled wire conductors that are parallel-wound with a common diameter and are separately coupled between a proximal connector element and to a distal electrode or terminal in the manner described in commonly assigned U.S. Pat. No. 5,007,435, for example. The coaxial construction technique may also be combined with the parallel-winding technique to multiply the total number of separate coiled wire conductors accommodated within a specified endocardial lead body outer diameter.
All of the above considerations as to the complexity of the leads, the number of leads implanted in a common path and the advancement of coronary sinus leads deep in the coronary veins have led to efforts to at least not increase and optimally to decrease the overall diameter of the cardiac lead body without sacrificing reliability and usability. More recently, it has been proposed to diminish the lead body further by eliminating the lumen for receiving the stiffening stylet and by replacing the coiled wire conductor with highly conductive stranded filament wires or cables. In bipolar or multi-polar leads, each such cable extends through a separate lumen of the lead body to maintain electrical isolation. Without the stiffening stylet, it is necessary to resort to use of another mechanism to pass the lead through the vessel paths identified above and to position and fix the distal end electrode of the lead at the desired site in the heart chamber or vessel.
One approach for implantation of such leads is disclosed in commonly assigned U.S. Pat. No. 5,246,014, incorporated herein by reference, that employs a highly flexible, introducer catheter surrounding the lead body and engaging a distal screw-in electrode. The assembly is advanced to the desired site and rotated to screw the distal screw-in electrode into the myocardium of the right atrium or ventricle. The introducer can also be used to direct a cardiac lead into the coronary sinus opening.
One difficulty with use of such an introducer surrounding the cardiac lead is that permanently implantable endocardial leads are formed typically with a proximal connector end assembly having a diameter exceeding that of the lead body and conforming to an industry standard so that the connector end assembly can be fitted into and seal with an IPG connector bore conforming to the same standard. Consequently, the introducer has to be made large enough to fit over the enlarged diameter connector end assembly. This detracts from the ability to advance the introducer and lead assembly through small diameter blood vessels. Or the lead has to be made with a small diameter, non-conforming, connector end assembly or without any connector end assembly and therefore requiring connection to an adapter to be made conforming to the standard. This is inconvenient and can result in a diminished reliability.
Another approach is disclosed in U.S. Pat. No. 5,003,990, also incorporated by reference herein, that relies on a guidewire and a carriage that releasably engages the distal screw-in electrode and is pushed along the guidewire as the lead body is pushed along the transvenous path. The guidewire is first introduced along one of the above-described desired paths, and the carriage engaging the distal electrode is placed over the proximal end of the guidewire and introduced into the blood vessel. Force is exerted against the lead body to push the carriage and the distal end of the lead body distally along the guidewire until the distal electrode is near to the desired site. The guidewire end or a curve in its distal end is engaged by the carriage, and the lead body distal end is bent until the screw-in electrode disengages from the carriage. Then, the carriage is retracted along the guidewire by pulling on another wire attached to the carriage or by the retraction of the guidewire, and the lead body is rotated from the proximal end to screw the helix electrode into the myocardium. Such retraction of the relatively bulky carriage presents the possibility of damage to an artery or vein by the carriage. In addition, releasing the carriage from the guidewire requires the lead or catheter to be bent in the area of the carriage presenting further possibilities of tissue damage during such carriage removal. Still further, once the lead is free of the carriage, its distal end will not necessarily be at its desired final implantation position. Use of the carriage during both implantation of the lead or catheter and retraction of the carriage requires numerous manipulations by the physician which only adds further complexity to an already complex procedure.
In a further approach disclosed in U.S. Pat. No. 5,304,218, incorporated by reference herein, a cardiac lead is formed with a channel in the distal tip that receives a guidewire that has already been advanced through the path to the cardiac implantation site. The lead is pushed over the guidewire to the cardiac implantation site where the guidewire is withdrawn and the lead is either fixed in place or left at the cardiac implantation site. There is no disclosure of how this approach could be used to advance a cardiac lead having an active or passive fixation mechanism at or near the channel in the distal end of the lead body.
Moreover, in both of these approaches, the lead body must possess sufficient column strength to allow it (as well as the carriage of the '990 patent) to be pushed from the proximal end outside the patient's body and along the guidewire. The lead body diameter and/or construction materials that are required in order to make the lead body stiff enough to accomplish this over the wire advancement method necessarily make the lead body larger and less flexible than is desirable to withstand the rigors of chronic flexing as described above. The over the wire approach is classically employed in advancement of balloon catheters for PTCA use which is intended to be of short duration.
Thus, a need remains for an introduction arrangement that allows the introduction of an endocardial cardiac lead having a highly flexible lead body and small lead body diameter into a heart chamber or vein and allowing the use of the active or passive fixation mechanism (if present) at the desired site.